I always thought it was a bad thing to put a banana in the fridge. At least the outside turns black almost immediately (well, within 12-24 hours anyway). But I don't know if that preserves the pulp or not. I'm not a great fan of banana eating. I throw out all black bananas. I can imagine that putting anything at lower temperatures would slow down or even stop many enzyme reactions. If you can stop fruit pulp maturation by inhibiting enzymes, I suppose that could explain it--but I really don't know anything specific. I'm going to have to sacrifice a banana to the fridge next bunch I buy and see if it really does preserve anything.

The cold temperature of your fridge encourages an enzyme (polyphenyl oxidase) naturally found in the banana to polymerise phenols in the banana skin into polyphenols . Polyphenols are similar to melanin, the pigment responsible for the colour in our skin. This is what blackens the skin of the bananas.
Dispite the color, the cold temperature will keep your banana firmer then a banana that was left at room temperature for the same amount of time. The enzymes that break the starch into sugar(which makes the banana soft and ripe) work better at room temperature.

If you want to make other fruit ripen quickly, stick the unripe fruit in a bag with a blackened fridge banana. The ethylene being given off by the damaged banana skin will facilitate the quick ripening of other fruits.

I always wondered why that happened! Thanks for the explanation. I guess I could go look it up, but is the polymerization reaction "normal" or is it something that only happens in the cold? Curious. Now I'm going to have to go see if there is a structure for the enzyme and what is known about the mechanism.